Questions: Phospholipids have both hydrophobic and hydrophilic ends. How does this structure make the phospholipid ideally suited to the selectively permeable structure needed by the plasma membrane?

The answer is the first one: The hydrophobic tails create a boundary that prevents water-soluble polar molecules from crossing the plasma membrane but allows lipid-soluble nonpolar molecules.

Explanation for each option:

1. The hydrophobic tails create a boundary that prevents water-soluble polar molecules from crossing the plasma membrane but allows lipid-soluble nonpolar molecules.

   • This is correct. Phospholipids have hydrophobic tails that face inward, away from water, forming a barrier that prevents water-soluble polar molecules from easily passing through the membrane. However, lipid-soluble nonpolar molecules can pass through this hydrophobic region.

2. The hydrophobic heads create a boundary that prevents water-soluble polar molecules from crossing the membrane.

   • This is incorrect. The hydrophobic part of the phospholipid is the tail, not the head. The heads are hydrophilic and face outward, interacting with the aqueous environment.

3. The hydrophobic heads create a boundary that prevents lipid-soluble nonpolar molecules from crossing the membrane.

   • This is incorrect. As mentioned, the heads are hydrophilic, not hydrophobic, and they do not prevent lipid-soluble nonpolar molecules from crossing the membrane.

4. The hydrophilic heads create a boundary that prevents water-soluble polar molecules from crossing the membrane.

   • This is incorrect. The hydrophilic heads are oriented towards the aqueous environment and do not prevent water-soluble polar molecules from crossing; rather, they interact with water.

In summary, the structure of phospholipids, with hydrophilic heads and hydrophobic tails, is ideally suited for forming the selectively permeable plasma membrane, allowing certain molecules to pass while restricting others.